Various processes allowing an intermediate structure, comprising in succession, a surface semiconductor layer, a dielectric layer, and a carrier substrate, to be formed are known from the prior art. It may, for example, involve a layer-transfer fabrication process (such as the processes known by the names SMART CUT® or ELTRAN™) or even the oxygen-implantation fabrication process (known by the acronym SIMOX: Separation by Implantation of Oxygen).
This intermediate structure, during a following finishing step, undergoes various treatments in order to convert the surface layer into a useful layer having all the expected properties especially in terms of average thickness, thickness uniformity, roughness, crystal quality, etc.
These known processes are especially employed for the fabrication of silicon-on-insulator (SOI) structures. In this case, the surface layer, which will become the useful layer, and the carrier typically consist of silicon and the dielectric layer of silicon dioxide.
These SOI structures must satisfy very precise specifications. This is especially the case for the final roughness of the useful layer and the thickness uniformities of the useful layer and of the underlying dielectric layer. Satisfaction of these specifications is required for the good operation of the semiconductor devices that will be formed in and on the useful layer.
Among the standard finishing treatments applied to an intermediate structure, smoothing annealing treatments that consist in exposing the surface layer to an inert or reducing atmosphere brought to a high temperature, typically above 1100° C., are known. This treatment, inter alia, allows, by surface reconstruction, the roughness of the layer exposed to the high-temperature atmosphere to be decreased.
These annealing operations may be carried out in furnaces suitable for treating a plurality of SOI structures simultaneously, under a controlled inert or reducing gas flow in order to promote the thermal homogeneity of the furnace. This gas is filtered in order to be extremely pure (less than 1 ppm of impurities) as any contaminant (O2, H2O, CO2, etc.) capable of reacting with the silicon disrupts the surface smoothing. Imperfect smoothing is characterized by a degree of residual or non-uniform roughness at the surface of the useful layer.
Roughness measurements are generally carried out using an atomic force microscope (AFM). With this type of apparatus, the roughness is measured on surfaces scanned by the tip of the AFM microscope, ranging from 1×1 μm2 to 10×10 μm2 and less commonly 50×50 μm2, or even 100×100 μm2. It is also possible to measure the surface roughness by other methods, in particular by means of a “haze” measurement. This method has, in particular, the advantage of rapidly characterizing the uniformity of the roughness over the entire surface of the useful layer. This “haze,” measured in ppm, is derived from a method that uses the optical reflectivity properties of the surface to be characterized, and corresponds to an optical signal scattered by the surface, owing to its microroughness. It is specified that the “haze” values that will be disclosed in this text are expressed in arbitrary units and were measured according to the same protocol and by the same device, in this case by an instrument of KLA Tencor SURF SCAN® SP type.
In the high temperature and annealing time ranges used for smoothing the surfaces of the SOI structures having a thin surface layer, a phenomenon of dissolution of the underlying oxide layer is capable of occurring. The dissolution phenomenon is in particular reported in the document “Novel trends in SOI technology for CMOS applications” by O. Kononchuk et al., that appeared in the journal Solid State Phenomena, volume 156-158 (2010) pp. 69 to 76. This document specifically explains that, in the high-temperature inert or reducing treatment atmosphere, the oxygen atoms of the dielectric layer are capable of diffusing through the surface layer and of reacting with the surface thereof in order to produce volatile species (gaseous silicon monoxide (SiO)), which are evacuated into the atmosphere of the furnace by the inert gas flow. This document also explains that for SOI structures having a thin surface layer, the diffusion of oxygen through the surface layer is limited by the ability to evacuate volatile species from the surface of the structure, and therefore that the extent of the dissolution phenomenon is locally linked to the gas velocity of the atmosphere of the furnace in the vicinity of the surface.
If the gaseous silicon monoxide (SiO) accumulates at the surface of the superficial layer during the heat treatment, it therefore locally slows down the dissolution, leading to differences in thickness of the surface layer and of the dielectric layer on the final product, which is particularly damaging. In order to limit the local accumulation of SiO it is necessary to maintain a high gas flow in the furnace, since the SiO is precisely evacuated via the inert or reducing gas flow circulating in the furnace.
The applicant has observed that, despite the controlled uniformity and purity of the gas flow in the furnace during a smoothing heat treatment at high temperature and under a high inert or reducing gas flow, certain SOI structures have “haze” levels greater than the expected level. This is in particular true at the edges of these structures, as illustrated in FIG. 1. Peripheral zones 100 of high residual roughness are present on the useful layer: they are shown on the “haze” mapping of FIG. 1 by darker zones, representative of a greater roughness (note that the central part of the mapping has darker regions in the form of butterfly wings, also known as “haze cross,” which do not correspond to a greater roughness but to an artifact of the measurement method used). The peripheral zones 100 are a problem for the final product for which the surface roughness and its uniformity at the surface of the SOI structure are key parameters.
Thus, a smoothing annealing operation according to the prior art makes it possible to generally smooth the surface of the SOI structures, guaranteeing a good thickness uniformity of the final SOI structures (owing to a sufficient evacuation of the volatile species linked to the dissolution) but generates, on certain treated structures, peripheral zones 100 of residual roughness that are incompatible with the roughness specifications of the final product.